Light-emitting diodes (LEDs), particularly white LEDs, have increased in size in order to provide the total light output needed for general illumination. As LED technology has advanced, the efficacy (measured in lumens/Watt) has gradually increased, such that smaller die now produce as much light as was previously created by emission from far larger die areas. Nonetheless, the trend favoring higher light outputs has led to larger semiconductor LED die sizes, or, for convenience, arrays of smaller die in series or series-parallel arrangements. Series arrangements are generally favored because the forward voltage of LEDs varies slightly, resulting, for parallel arrangements, in an uneven distribution of forward currents and, consequently, uneven light output.
Ordinary light sources commonly have a fixed light-distribution pattern that cannot be modified by the user. The beam angle of the light emanating from the light source depends on the intended application; in the retail marketplace, for example, “spotlights” refer to narrow-beam sources while “floodlights” illuminate over a wide area. While the technology for varying beam angle is well known, the resulting systems tend to be too costly or inefficient for consumer use. Movable refractive optics, for example, can be used to alter beam angle as the position of a lens is varied. But the acceptance angle of such optical systems varies with position, so the efficiency decreases as the beam angle is altered. Moreover, because multiple optical surfaces are required, light losses can quickly mount as additional optical elements are added. Preventing color separation, distortion and other artifacts may require still further optical features.
Yet the ability to vary beam angle may be desired in various applications where expensive optical systems would not be cost-justified. A merchant, for example, may wish to vary the output of the same display light source to illuminate an array of objects or a single, small object. A need, therefore, exists for cost-effective light sources that produce variable beam angles with uniform illumination and without sacrificing beam quality.